In order to improve fuel economy during low load conditions, some engines may be configured to operate in a selective cylinder deactivation mode where one or more cylinders of the engine are deactivated via disabling of intake and/or exhaust valve actuation, interruption of fuel injection, and/or disabling of spark ignition to the deactivated cylinders, for example. During operation in the selective cylinder deactivation mode, also referred to as “skip fire,” the total engine fuel amount may be redistributed to the fired cylinders, increasing per-cylinder load and reducing pumping work, thus increasing fuel economy and improving emissions. The cylinder(s) selected for deactivation may change with each engine cycle, such that a different cylinder or combination of cylinders is deactivated per engine cycle. Further, the number of cylinders deactivated per engine cycle may change as engine operating conditions change.
In engines with port fuel injection (PFI) systems, air-fuel ratio control during skip fire may be challenging due to the delay between when the fuel is injected and when the air charge is calculated. Specifically, in PFI systems, fuel for a cylinder is generally injected while that cylinder's intake valve is closed, to provide for desired evaporation and mixing of the fuel. However, the amount of air trapped in that cylinder is determined up to two engine revolutions later, after the intake valve opens and closes again. With a skip fire strategy, the intake manifold dynamics may change dramatically in that time (e.g., the engine may transition into or out of skip fire operation), resulting in a different air charge than originally predicted, and therefore a different air-fuel ratio than desired.
One approach to improving air-fuel ratio control in skip fire engines includes injecting fuel via a direct injection (DI) system, because DI injection can occur much later, when an updated air charge calculation is available. However, the inventors herein have recognized that at part load conditions, PFI offers better efficiency than DI due to improved air-fuel mixing, lower pumping work, and lower fuel pump parasitic losses. Thus, operating with only DI may degrade fuel economy.
In light of the above issues, the inventors herein have devised an approach to maintain the fuel economy benefits of port injection while providing increased air-fuel ratio control during skip fire operation of an engine. In one embodiment, a method comprises, during a skip fire mode, port injecting a first fuel quantity to a cylinder of an engine, the first fuel quantity based on a first, predicted air charge amount for the cylinder and lean of a desired air-fuel ratio, and direct injecting a second fuel quantity to the cylinder, the second fuel quantity based on the first fuel quantity and a second, calculated air charge amount for the cylinder.
In this way, a majority of the fuel provided to the cylinder may be injected via port injection, to provide enhanced fuel vaporization and mixing, and lower pumping work. The amount of fuel injected via the port injection may be deliberately lean of a desired air-fuel ratio, where the desired air-fuel ratio is calculated based on an estimated air charge amount for that cylinder. Then, later in the engine cycle, when the actual amount of trapped air charge in the cylinder can be calculated, an extra amount of fuel may be provided via direct injection to bring the overall air-fuel ratio to the desired air-fuel ratio.
The present disclosure may offer several advantages. For example, by injecting a majority of the fuel via port injection, desired fuel economy may be maintained. By providing a “make-up” injection of fuel later in the engine cycle via direct injection, a desired air-fuel ratio may be maintained, even as intake manifold pressure and charge air flow changes due to operation in the skip fire mode.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.